This invention relates to a slag-retaining plug for use in steel making and, more particularly, to a floatable plug which floats in a predetermined position in the slag layer and partially in the underlying molten steel in a predetermined position to minimize vortexing upon tapping of the molten steel and to automatically close the tap hole prior to any draining of slag from the tap hole.
In the production of steel, whether by pneumatic, open hearth, or electric furnace techniques, and whether the heat of steel is poured into ingot molds or is continuously cast, the steel is transferred by ladles or tundishes during the steel making operation. The ladle has a shell which is fabricated from steel plate and is lined with a refractory brick to withstand temperatures in excess of 2000.degree. F. In the case of open hearth steel making processes, the tap hole of the furnace is opened and the steel heat is emptied into the steel ladle through a tapping spout. The necessary slag cover which, due to its density, floats on the steel overflows the ladle through a slag spout on the ladle and is either received in a slag pot or is merely permitted to run into the cellar of the charging floor for removal. Some slag remains on the steel as the heat is tapped through a pouring nozzle at the bottom of the ladle. The pouring nozzle is typically controlled by a sliding gate or stopper rod assembly. A sliding gate comprises a refractory slab which is mounted below the pouring nozzle for horizontal reciprocation to cut off flow from the nozzle. A stopper rod assembly comprises a refractory rod extending through the bath to the pouring nozzle. The stopper rod includes a ladle rigging for raising and lowering the stopper rod, and comprises a steel shaft which is insulated by fire clay sleeves encircling the rod. The fire clay sleeves must be replaced after a number of heats to prevent the mounting rod from being in contact with the relatively high temperatures. This operation effectively removes a ladle from production until this operation can be completed.
The ladle may be used to transfer molten steel from the steel making furnace and pour it at a controlled rate into continuous casting equipment or into ingots. Where the ladle is employed to feed molten steel to a continuous casting machine, the ladle pours the steel through its bottom opening into a tundish, which serves as a reservoir for the continuous casting mold.
Whether the ladle is employed to supply molten steel to ingot molds or to a tundish, it is essential to terminate the pour prior to the discharge of molten slag from the ladle. Obviously, the inclusion of slag in ingots or in the continuously cast strand results in defects in the finished product, and the presence of slag is extremely corrosive to the tundish lining.
The precise time at which slag may enter the tap hole in the ladle is not merely a matter of observing the slag level in the ladle, since the molten metal and slag tend to form a vortex above the tap opening, and this vortex tends to draw slag into the molten steel exiting the tap hole even though there may exist a significant amount of steel in the ladle. As a precaution, steel makers tend to terminate the pour well prior to pouring all of the steel from the ladle. For example, it is typical practice when pouring a 400,000-lb. heat to terminate pouring with 10,000 lbs. of steel still in the ladle.
The problem of slag inclusion while pouring from a steel making vessel itself, such as a pneumatic steel making vessel as opposed to a transfer ladle, also exists. Early steel making processes, such as the Bessemer process, involved tilting the vessel to pour off the slag, following by further tilting to pour the steel heat. This operation presented numerous difficulties, since much depended upon the skill of the operator. Moreover, slag and steel tend to build up on the lip of the vessel, further complicating the pouring operation.
An alternative approach to pouring over the lip of the vessel is to utilize a tap hole in the sidewall of the vessel. The tap hole is closed during the course of the heat and is opened at the end of the heat. The tap hole is located below the slag layer so that the steel may be removed by progressively tilting the vessel and allowing the slag to float on the molten steel. A major problem in a side tap operation (or, for that matter, a bottom tap operation) is the vortex formed by the molten liquid, which tends to draw the slag through the tap hole during pouring.
Solutions have been proposed which are intended to maximize the amount of steel drawn from a ladle or steel making vessel while retaining the slag, and to minimize the vortexing problem. One such solution is proposed in U.S. Pat. No. 4,526,349, which discloses the use of a spherical heat-stable material having a density such that it floats substantially at the interface between the slag and the steel and the use of a surrounding ring which is substantially toroidal in shape. Preferably, the discrete spherical piece is positioned within the open center of the toroid so that the spherical refractory may be drawn into and plug the tap hole while the surrounding toroid will minimize vortexing and act as a dam for the slag. While this arrangement may effectively control the vortexing problem, the use of the discrete elements poses a handling problem for the caster in positioning a spherical object within a toroidal object during the melting and tapping operation.
In U.S. Pat. No. 4,494,734, there is disclosed a device for the retention of slag during the tapping operation which comprises a cone-shaped refractory having a specific gravity lower than that of the steel in the converter but higher than that of the slag. The device is added to the melt just prior to the formation of a vortex above the tap hole, and is intended to partially plug the tap hole prior to any tapping of slag. It has been determined, however, that the cone-shape disclosed in that patent will not necessarily float in the desired orientation for plugging purposes, but will, rather, be randomly oriented during the pouring operation, since its center of buoyancy is at or very near its center of gravity.
Further examples of closure plugs for ladles or converters may be found in U.S. Pat. Nos. 4,462,574; 3,124,854; 3,934,755; 2,718,398; and 4,390,170.